Intermittent ignition failure at above idle RPM

[Big Boat Bay Buster]

I'm going to post a similar problem you have. I have a Thunderbolt IV ignition, 1987 V8. After cruising for 20 minuets or so, it bogs down. Idles fine, but wont go above 1200 RPM. After I shut the engine down and let it cool, it runs fine. I'm convinced its in the ignition system, notably the TB module (box on the exhaust manifold), but I want to be sure.

 

[jakwi]

Yeah that is similar. I had an issue like that when I first bought the boat would run fine for 20 min then stall, and get progressively more difficult to restart. The fix for me was a ignition coil.

They say the thunderbolt ignition module is all but bullet proof, not to mention it is really expensive. I'd be tempted to replace the ignition system with something else if the thunderbolt died.

For me heat doesn't seem to have anything to do with it. Mine will fail in the way described cold or hot.

After my last post I decided to perform the knock sensor/module test. Unfortunately/fortunately though it is working fine. Exactly as described in the manual.

I also inspected my plug wires to see if I could find any abrasions, and I cleaned the distributor cap, I figured it couldn't hurt.

The only thing left that I can think of is the ignition sensor, the ignition module, fuel pressure verification, and maybe timing check?

I'm really running out of ideas.

 

[jakwi]

You know you could always try mounting the module somewhere that doesn't get as hot, or pick up a can of freeze spray to cool the module when it starts acting up.

 

[jakwi]

As I was reading the manual I found a reference to a test for the ignition sensor. It states to disconnect the two wires from the sensor and check the resistance. If less than 100 ohms replace.

I measured between the white with green stripe, and the white with red stripe wires. I have 6.7 Megaohms

Can I talk anyone into measuring their sensor so that I know what a known good one reads?

It seems like mine passes, but 6.7 Mohms is a long way from 100 ohms

 

[jakwi]

This morning I looked at the anti siphon valve. Outside it's a little crusty, but inside it was clean as a whistle. The checkball moves freely. I'm not sure how to check the fuel pickup. I have a poly tank down in the bilge, The anti siphon valve threads into an elbow that feeds into the tank. If I remove that elbow will it pull up the feed line with it?

This week I will put together the pressure/vacuum gauge I found in the HOW TO: Testing your boats fuel system. My plan is to water test next friday. Any ideas on what specific tests I should try would be greatly appreciated.

 

[achris]

...I've already tried disconnecting the knock sensor, but maybe the Knock control module is falsely indicating knock at 4k, Maybe it is sending the signal to retard ignition when not needed..? Seems like I can't simply remove the module because it sends an 8 to 10 v signal to the ignition module when no knock is detected.

IF the knock module is present, then yes, it sends a 8-10 volt signal when no knock is present. When knock IS present, then the knock module DRIVES that voltage down. With no module present (or it unplugged) that particular signal line 'floats' and the ignition module ignores it. It's the same wire that you put the module into base timing mode by grounding.

Remember, the early V6 and 5 litre engines were supplied without knock sensors. Their ignition modules coped just fine.

Chris......

 

[jakwi]

I'll try both base timing mode, and simply unplugging the knock sensor, but I believe the knock sensor is working properly. I performed the test from the manual and it responded exactly as it should.

I also tried to pull the feed tube from the fuel tank, but the pickup is to large to fit through the hole, so I am unable to pull it out and inspect it. I did blow into it, and it didn't feel like there was any restriction.

For the water test I will check the fuel system with a vacuum/pressure gauge as described in the how to.

Any other suggested tests I can perform on the water to narrow down the root cause would be appreciated.

 

[jakwi]

This weekend's update:
I performed part of the fuel test.
At idle there is about 2" Hg draw on the fuel tank. I don't know why, but I didn't think to try it at a higher RPM.
If I pinch off the tank side the draw increases to 15 or 16 " Hg for at least 20 seconds, although it probably took 20 seconds to climb that high.

I am still working on a way to measure pressure at the carb.

I tried base timing mode, No change
I tried unplugging the knock sensor, No change
I sprayed soapy water all over the distributor, and looked for a short around the distributor and sparkplug leads. nothing visible or audible.
I removed each sparkplug lead one by one, and cleaned them with soapy water, and reinstalled. They were a little oily, but otherwise no damage noted, and now they are dry and clean. No change.
I cleaned the grounds where the knock sensor, wiring harness, and other major grounds are tied to the Motor. They looked fine, but in any case, no change.

I tested the ignition sensor one more time, this time while totally cold. in one direction it measures open, in the other it measured 9.5 Megaohms.

Yet to do
- Measure the pressure at the carb
- Replace cap and rotor, will arrive this afternoon
- Possibly try an alternate tank feeding the pump/or try measuring the vacuum at a higher level than idle

Questions

- Can I measure spark plug lead resistance effectively?
- Is there a rev limiter for the Thunderbolt V that is lower when the motor is not loaded
- The reason I ask is that it acts a lot like a rev limiter is hitting
- The same day I brought it back from the lake it seems like it was having a hard time at a lower RPM, maybe 2800 or 3k, but now it is consistent at 4k. At the time I thought it was an improvement, but not solved. Now I'm starting to wonder if I fixed it but I'm just hitting the rev limiter while unloaded. I really need to get it in the water, but I can't seem to get there. Maybe I'm just loosing my mind.


As always thoughts and opinions are appreciated.

 

[jakwi]

Well I feel like a world class idiot, but I'm going to throw this out there so that whomever has been following this knows the answer.

I finally got it out on the water with the intention to measure the fuel draw vacuum under load. To make sure that there was no obstruction in the pickup. Turns out it is about 3" of Hg at 3800 RPM, so that is good.

Honestly I was completely certain that the issue was still present.

My son and I took it out and what do you know it popped right up on plane and ran great at about 3/4 to 7/8 full throttle. I've never really run it this fast, but I was testing.

RPM indicated 3800 rpm. If I pushed it to full throttle it started cutting out just past 4000 rpm. Exactly like on the trailer. But honestly I never have ever pushed it to full throttle at the top end of the range like that. As could be expected it took a lot less throttle to get there unloaded.

So anyway, it turns out I have been bouncing off the rev limiter for all this time.

My last statement in my previous post was right on the money except that I now don't think it's an unloaded rev limiter.

The same day I brought it back from the lake it seems like it was having a hard time at a lower RPM, maybe 2800 or 3k, but now it is consistent at 4k. At the time I thought it was an improvement, but not solved. Now I'm starting to wonder if I fixed it but I'm just hitting the rev limiter while unloaded. I really need to get it in the water, but I can't seem to get there. Maybe I'm just loosing my mind.

When I first bought the boat I read somewhere that Faria tach's were inaccurate, So I attached an external tach to verify mine. I found it was off by about 200 RPM at idle. I pulled the Faria tach and changed the setting so that it indicated more closely at idle.

It turns out that while I corrected the indication at idle, I shifted the top end significantly. IE I think while my tach was indicating 4k my engine was actually turning 5k.

I haven't actually verified the top end of the tach, but I'm sure it is running normally, and that I have performed a ton of preventative maintenance that didn't need to be done. Oh well. Live and learn. Thanks again for all of the help.

 

[achris]

Might be worth investing in a cheap digital tacho and mount it on the instruments if you need an accurate engine speed reading....

 

[jakwi]

Something like this? Are they accurate?